Glyphosate, a herbicide, is the controversial active ingredient in popular weed killers such as Roundup.

Bees that came in contact with the herbicide glyphosate in the study had lower levels of healthy gut bacteria that helps digest food and ward off sickness and were more likely to die from pathogens. About half of the healthy bees survived the introduced pathogen Serratia marcescens. Only a tenth of bees affected by glyphosate survived.

“We need better guidelines for glyphosate use, especially regarding bee exposure, because right now the guidelines assume bees are not harmed by the herbicide,” Erick Motta, a graduate student who led the research, said in a statement. “Our study shows that’s not true.”

Motto said weed killer isn't solely responsible for a declining bee populations, but is a contributing factor enough that "people should worry about," especially because glyphosate is widely used.

Human health effects of glyphosate have been widely debated. The World Health Organization has determined that glyphosate is "probably carcinogenic to humans," but Monsanto (the company behind weed killer Roundup) says it's not a probable carcinogen.